In statistics, the **null hypothesis** is the hypothesis that there is no relationship between two measured phenomena. It is the hypothesis that something is not present, that a treatment has no effect, or that there is no difference between two parameters. The null hypothesis is usually considered true until proven false. It is denoted **H _{0}**, pronounced

*H-nought*,

*H-null*, or

*H-zero*.

The most important requirement for a null hypothesis is that it must be falsifiable. In other words, it must be possible to compute the probability that the test rejects the null hypothesis when the null hypothesis is true. This probability is called the test’s **significance level**.

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